The present invention relates to a composite material for various devices and apparatuses, especially a composite material having high thermal conductivity and a low coefficient of thermal expansion (hereinafter called CTE), which material is useful for a heat-dissipating substrate of semiconductor devices.
Recently, market demands for faster computation and higher integration of semiconductor devices (various devices incorporating semiconductor elements; the same shall apply hereinafter) have been rapidly increasing. At the same time, a heat-dissipating substrate for mounting a semiconductor element (hereinafter also called simply a heat-dissipating substrate) of a semiconductor device has been required to further increase its thermal conductivity in order to dissipate the heat generated by the element more effectively. The substrate has also been required to have a CTE much closer to that of the semiconductor element and of the device""s other members (the peripheral members) adjacent to the substrate in order to further decrease the thermal strain between the substrate and the semiconductor element and between the substrate and the peripheral members. Specifically, Si and GaAs, which are commonly used as a semiconductor element, have a CTE of 4.2xc3x9710xe2x88x926/xc2x0 C. and 6.5xc3x9710xe2x88x926/xc2x0 C., respectively. An alumina ceramic, which is commonly used as an enclosing material of semiconductor devices, has a CTE of about 6.5xc3x9710xe2x88x926/xc2x0 C. Consequently, it is desirable that a heat-dissipating substrate have a CTE close to these values.
As electronic devices expand their field of application significantly in recent years, semiconductor devices are further diversifying their application fields and performances. In particular, market demands are increasing for semiconductor power devices, such as high-output ac converters and frequency changers. In these devices, a semiconductor element generates heat no less than several to several-dozen times that of a semiconductor memory or LSI; such a semiconductor element usually generates tens of watts. Therefore, heat-dissipating substrates for these devices are required to have significantly increased thermal conductivity and a CTE much closer to that of the peripheral members. To meet this requirement, semiconductor power devices have the fundamental structure described below. First, an Si semiconductor element is placed on a first heat-dissipating substrate made of an aluminum nitride (hereinafter also called simply AlN) ceramic, which is excellent in electrical insulation quality and thermal conductivity. Second, a second heat-dissipating substrate made of a highly heat-conductive metal, such as copper or aluminum, is placed underneath the first heat-dissipating substrate. Finally, a radiator that can be cooled by air or water is placed underneath the second heat-dissipating substrate. The generated heat is dissipated promptly to the outside by such a structure. Consequently, the second heat-dissipating substrate is required to conduct the heat received from the first heat-dissipating substrate promptly to the radiator underneath. This means that it is essential for the second heat-dissipating substrate to have not only high thermal conductivity but also a CTE close to that of the first heat-dissipating substrate. Specifically, the second heat-dissipating substrate is required to have a CTE as small as 6xc3x9710xe2x88x926/xc2x0 C. or less.
Such a heat-dissipating substrate has been made of a composite alloy consisting mainly of W or Mo, for example. The drawback of these substrates, however, is that they are heavyweight as well as high cost owing to the costly material. To resolve this problem, various aluminum (hereinafter also called simply Al) composite alloys have been attracting attention in recent years as material that is light in weight as well as low in cost. In particular, Alxe2x80x94SiC-based composite materials, consisting mainly of Al and silicon carbide (herein-after also called simply SiC), are comparatively low in material cost, light in weight, and high in thermal conductivity. A pure-Al single body and a pure-SiC single body, which are usually available in the market, have a density of about 2.7 g/cm3 and about 3.2 g/cm3, respectively, and a thermal conductivity of about 240 W/mxc2x7K and about 200 to about 300 W/mxc2x7K, respectively. A pure-SiC single body and a pure-Al single body have a CTE of about 4.2xc3x9710xe2x88x926/xc2x0 C. and about 24xc3x9710xe2x88x926/xc2x0 C., respectively. The adjustment of the constituting ratio of SiC and Al produces a wide CTE range. They are, therefore, advantageous in this respect.
As a result, various composite materials comprising Al and ceramic have been developed with particular focus on an Alxe2x80x94SiC-based composition. For example, the published Japanese patent applications Tokukaihei 8-222660, Tokukaihei 8-330465, Tokuhaihei 1-501489, and Tokukaihei 2-243729 disclose inventions in which composite materials are produced by the infiltration method or casting method. Another published Japanese patent application, Tokukaihei 9-157773, discloses an invention in which a composite material is produced by hot-pressing a formed body of mixed powders. The U.S. Pat. No. 5,006,417 discloses a composite material in which Si is added to the Alxe2x80x94SiC-based composition. The composite material has a metal matrix in which a reinforcement material insoluble in the metal matrix is distributed. For example, an Alxe2x80x94SiCxe2x80x94Si-based composite material is reinforced by SiC and Si. The object of that invention is to provide a lightweight member to be sandwiched between a plastic-glass-based circuit board having a large CTE and a ceramic member having a small CTE to be mounted on the board. The lightweight member is required to have an intermediate CTE between the two CTEs. The composite material of that invention has low density, small CTE, good thermal conductivity, good dimensional stability, and good formability. A typical composite material stated in that invention comprises a 40 to 60 vol % Al matrix in which 10 to 40 vol % Si, which is a semi-metal, and 10 to 50 vol % SiC are distributed. However, the preferable embodiment (the sample shown in Table 1 of that invention), containing Al with an amount of 45 to 55 vol % (the wt % is much the same), has a CTE of 8 to 9xc3x9710xe2x88x926/xc2x0 C. or so. Therefore it cannot be said that the composite material has a CTE close to that of ceramic, which has a CTE of 3 to 6xc3x9710xe2x88x926/xc2x0 C. or so. The composite material is obtained by solidifying a mixed powder comprising the foregoing constituents at a temperature higher than the melting point of the metal constituent. According to FIG. 2 of that invention, it can be conjectured that an Alxe2x80x94SiCxe2x80x94Si-based material produces a composite material having a CTE of 6xc3x9710xe2x88x926/xc2x0 C. or less when the amount of SiC or Si is about 70 vol % or more. That invention states that the Alxe2x80x94SiCxe2x80x94Si-based composition can produce a composite material having a thermal conductivity up to about 120 W/mxc2x7K if a proper composition is selected, although the proper composition is not stated. Other researchers and engineers have been studying materials obtained by the liquid-phase sintering method. A composite material in which Al is replaced by Cu is also advantageous from the foregoing stand-point. In the present invention, the above-described materials are hereinafter called the first-group composite materials.
As described above, the first-group composite materials have high thermal conductivity. However, because Al and Cu have a large CTE, a composite material having a CTE as low as about 6xc3x9710xe2x88x926/xc2x0 C. cannot be obtained unless the amount of SiC, which has a small CTE, is increased. The increase in the amount of SiC, which has high hardness, makes it difficult to form the powder and to sinter the formed body. In addition, composite materials have been required to have a complicated shape, such as a fin shape, in recent years. These types consume much time and effort for the finishing process.
On the other hand, composite materials comprising silicon (Si), which has a small CTE and is lightweight in comparison with the first-group composite materials, and silicon carbide (SiC) have been developed. For example, the published Japanese patent application Tokukaihei 5-32458 discloses an Sixe2x80x94SiC-based composite material for a member to support the raw material for the formation of an Si or other semiconductor element when the raw material is heat-treated in the manufacturing process. The composite material is obtained by infiltrating molten Si into a porous body that is produced by sintering a highly pure SiC powder, having an iron content of 5 ppm or less, at a temperature between 1,500 and 2,300xc2x0 C. Another Sixe2x80x94SiC-based composite material that contains up to about 70 vol % SiC is reported in the Advanced Structural Inorganic Composites (1991), pp. 421 to 427. The composite material is obtained by reaction-sintering a formed body comprising a mixture of highly pure SiC powders and highly pure carbon (C) powders. Table II in the literature shows a composite material having a thermal conductivity as high as 186.6 W/mxc2x7K. However, no report has been found that demonstrates the example that these composite materials are applied to a heat-dissipating substrate or other members of semiconductor devices. In the present invention, these composite materials are hereinafter called the second-group composite materials.
An object of the present invention is to solve the foregoing problems that the first-group composite materials have and to improve the second-group composite materials so that both composite materials can be used effectively as members of semiconductor devices.
The composite material of the present invention (a) has a structure in which the interstices of a three-dimensional network structure comprising ceramic (a first constituent) are filled with a semi-metal-containing constituent (a second constituent) produced by deposition after melting, (b) has a CTE of 6xc3x9710xe2x88x926/xc2x0 C. or less, and (c) has a thermal conductivity of 150 W/mxc2x7K or more. The types of the composite material of the present invention include a composite material in which the ceramic contains silicon carbide (SiC) and a composite material in which the semi-metal is silicon (Si). The present invention encompasses (a) semiconductor-device members comprising the foregoing composite materials and (b) semiconductor devices comprising the above-mentioned members.